Accurate measurement of the angular motions of a spinning body contributes significantly to the development of experimental projectiles and rockets, and to the diagnosis of existing munitions and weapons systems. Such measurements can in some cases be made using high-speed photography but this technique is generally used for only limited portions of a projectile flight for reasons of both expense and practicability. Also, the precision of angular measurements is limited in this methodology. Another measurement technique used for obtaining angle of attack data is yaw cards but this technique is low resolution and provides only a small number of discrete data points along a trajectory. Some kind of on-board inertial angular rate sensor would seem a logical candidate for obtaining continuous data throughout a flight, but expense is often an issue and there are a host of problems associated with using such devices in high spin and high-g environments.
In developmental work, continuous in-flight angular orientation histories can be used for projectile aerodynamic characterization, test and evaluation of guidance and maneuver systems, and provide a truth measure for the test and evaluation of other pointing angle measurement systems, such as rate integrating inertial systems. The determination of the navigation pointing angle is of importance for the effectiveness of guidance and terminal seeking systems and advanced video imaging systems for target location, by way of example.
Restricted slit silicon solar cells have been used to indicate the solar attitude and roll rate of projectiles. A spinning projectile with optical sensors provides a pulse train, which when combined with calibration data, provides measurable quantities of the solar attitude and solar roll history. An optical sensor suitable for high-resolution solar attitude measurements is described in U.S. Pat. No. 5,909,275, which is hereby incorporated by reference. The variation in roll position of a tilted solar sensor when aligned with the solar plane is indicative of the angle between the axis of rotation of the projectile and the parallel light source. Using a variety of sensor orientations on a spinning body, a unique solution to the angle, "sgr"s, between the light source and the axis of rotation can be determined from a time-stamped history of solar alignment. Even though the angle between the axis of rotation and the solar vector can be determined, there are infinite orientations within the navigation system for which the angle, "sgr"s, has the same value.
In another development, described in U.S. patent application entitled xe2x80x9cMethod and System for Determining Magnetic Attitude,xe2x80x9d having inventors T. Harkins, D. Hepner and B. Davis, Ser. No. 09/751,925, filed Jan. 2, 2000 now U.S. Pat. No. 6,347,763, which application is hereby expressly incorporated by reference, a magnetic sensor array utilizes the outputs of one or more magnetometers, each having a sensitive axis, to obtain the orientation of the axis of rotation of a spinning body relative to a magnetic plane. The magnetic plane is defined by the body axis of rotation and a magnetic field vector. The angle between a magnetometer sensitive axis and the axis of rotation of the body is defined as lambda (xcex). With an array utilizing two magnetometer sensors at respective distinct and non-supplementary angles, xcex1 and xcex2, a unique determination may be made of "sgr"M, the angle between the magnetic field and the axis of rotation for the spinning body. However, like the solar sensor array described above, there are infinite orientations within the navigation system where the angle, "sgr"M, is a constant.
Accordingly, it is the primary object of the present invention to provide an arrangement, and a simple, robust methodology, wherein an on-board, multi-sensor system solution completely determines the orientation of an axis of rotation of a spinning body with respect to a convenient navigation system.
The present invention is a system and a methodology wherein a multiple field environment is utilized to determine the orientation of a spinning body within a convenient navigation coordinate system. An example is described containing a constellation of optical and magnetic sensors. Methodologies are developed for data processing to generate angular orientation in real-time or post-flight. Potential applications for the obtained data include determination of angular motion histories of experimental, developmental and tactical projectiles. The resulting angle data can be utilized with diagnostic tools for projectile aeroballistic characterization, determination of maneuver authority for guided munitions, and weapon/projectile/payload interaction analysis. The processed data can also provide a relative roll orientation and roll rate reference for calibration of on-board data sources such as accelerometers and angular rate sensors. Finally, the combination of magnetic sensors and on-board processing of data potentially provides navigation assistance for xe2x80x9cjammedxe2x80x9d GPS fitted munitions.
The determination of the orientation of a spinning body, that is, the pointing direction, is accomplished with first and second sensor arrays on board the body in flight. The first array is responsive to a first field, such as a solar field, represented by a vector having magnitude and direction. The array is utilized to obtain a value for the orientation of the axis of rotation of the body with respect to the first field direction, which is known. The second array is responsive to a second field, such as the earth""s magnetic field, represented by a vector having magnitude and direction. The second array is utilized to obtain a value for the orientation of the axis of rotation of the body with respect to the second field direction, which is also known. By vectorily combining the known and obtained values, the pointing direction may be determined.